The objectives of this investigation are to determine the mechanisms and the possible physiological significance of enzyme regulation by a diverse series of apparently distally related groups of intermediary metabolites. Phosphoenolpyruvate carboxylase of bacterial origin is subject to this type regulation. Its primary function is to make C-4 compounds for the citric acid cycle from a C-3 compound originating in glycolysis. It is activated by a precursor of that C-3 compound and also by the cosubstrate of the citric acid cycle reaction in which its product is a substrate. Aspartate, the transaminated product of this carboxylase reaction is an inhibitor of its activity, pyrimidine phosphates, an end product of one of the metabolic pathways of aspartate utilization, is an activator. This investigation will attempt to determine the kinetics of interaction of each of these compounds, singly and in combinations, with the enzyme, and through the use of genetically altered enzymes, the structural requirements for these diverse interactions.